The term “light” as used herein refers to any part of the electromagnetic spectrum, including but not limited to visible light and radio frequency.
Tuneable Distributed Bragg Reflector (DBR) lasers are used to provide accurate, controllable frequency laser light. The optical path length of the laser cavity defines longitudinal modes. The frequencies of the longitudinal modes are such that the round-trip phase for light of that frequency is an integer multiple of 2 pi radians. In a typical DBR laser this round trip phase can be adjusted by electronic or thermal control of a small length of the laser cavity known as a ‘phase section’ thereby altering the refractive index and therefore the optical path length locally in that section. Electronic control is achieved by injecting current into the phase section, and thermal control is achieved by altering its temperature.
In a DBR laser, frequency selectivity is achieved using a length of grating, known as a distributed Bragg reflector (DBR), at one end the cavity. When the frequency of the peak reflectivity of the grating (Bragg frequency) is aligned to the frequency of one of the longitudinal modes lasing can occur with high efficiency in that longitudinal mode and the laser is said to be operating at the mode centre. The Bragg frequency of the DBR may be adjusted by electronic or thermal control. When the Bragg frequency and the longitudinal mode frequency move apart (either due to changes in the DBR section, the phase section or other parts of the laser cavity over life) the laser will continue to lase at the longitudinal mode frequency, but reflection from the grating is no longer maximised.
The frequency is typically controlled by means of an external frequency reference, such as an etalon, used as a frequency locker. The laser frequency is corrected by adjusting some control parameter (such as the phase section current, or the overall chip temperature) that adjusts the optical cavity length and hence the longitudinal mode frequencies. Frequency lockers add complexity and cost to a device, and for some applications (for example where the frequency does not need to be controlled to very high tolerance) it would be beneficial to operate without the need for a frequency locker.
In addition, the performance of all sections of the laser is temperature dependent (indeed, this is the principle behind thermal control). This means that the operating temperature of a DBR laser must be consistent within a small range in order not to throw off the calibration. The cooling required to achieve this adds significantly to the cost and size of the DBR laser. Being able to provide a laser which is not dependent on operating temperature would be particularly useful for low-cost and low-space applications. For thermally controlled lasers, the operating temperature can be considered as the temperature of parts of the laser chip distant from the heating elements used for thermal control.